Cephalopod Hox genes and the origin of morphological novelties

Cephalopods are a diverse group of highly derived molluscs, including nautiluses, squids, octopuses and cuttlefish. Evolution of the cephalopod body plan from a monoplacophoran-like ancestor(1) entailed the origin of several key morphological innovations contributing to their impressive evolutionary success(2). Recruitment of regulatory genes(3), or even pre-existing regulatory networks(4), may be a common genetic mechanism for generating new structures. Hox genes encode a family of transcriptional regulatory proteins with a highly conserved role in axial patterning in bilaterians(5); however, examples highlighting the importance of Hox gene recruitment for new developmental functions are also known(6,7). Here we examined developmental expression patterns for eight out of nine Hox genes(8) in the Hawaiian bobtail squid Euprymna scolopes, by whole-mount in situ hybridization. Our data show that Hox orthologues have been recruited multiple times and in many ways in the origin of new cephalopod structures. The manner in which these genes have been co-opted during cephalopod evolution provides insight to the nature of the molecular mechanisms driving morphological change in the Lophotrochozoa, a clade exhibiting the greatest diversity of body plans in the Metazoa.